The Royal Law
But why and how is loving one's "neighbor" an actual Law of God? The Messiah answered the question:
"22:35 Then one of them, which was a lawyer [see Scribes], asked him a question, tempting him, and saying, 22:36 Master, which is the great commandment in the law?
"For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself"
To "love thy neighbour as thyself" did not originate in the New Testament. Jesus Christ ("10:1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; 10:2 And were all baptized unto Moses in the cloud and in the sea; 10:3 And did all eat the same spiritual meat; 10:4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ" 1 Corinthians 10:1-4 KJV; see also 'The God Of The Old Testament' and Christ The Creator) gave it to His people long before, as recorded in Leviticus:
"19:13 Thou shalt not defraud thy neighbour, neither rob him: the wages of him that is hired shall not abide with thee all night until the morning. 19:14 Thou shalt not curse the deaf, nor put a stumblingblock [see Stumbling Blocks] before the blind [which includes, moreover, those who are as yet spiritually blind - see When Their Eyes Were Opened and The Cure For The Carnal Mind], but shalt fear thy God: I am the LORD. 19:15 Ye shall do no unrighteousness in judgment: thou shalt not respect the person of the poor, nor honour the person of the mighty: but in righteousness shalt thou judge thy neighbour. 19:16 Thou shalt not go up and down as a talebearer among thy people: neither shalt thou stand against the blood of thy neighbour: I am the LORD. 19:17 Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him. 19:18 Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD." (Leviticus 19:13-18 KJV)
Many true Christians began as un-Christian, or even fiercely anti-Christian (e.g. the apostle Paul - see On The Road To Damascus), but when their time of awakening came (again, see The Cure For The Carnal Mind) and if they then truly repented, they were washed clean of all that happened before. For that reason, Christ commanded His people to not only "love your neighbor," but to also "love your enemies" because they may some day be your brothers and sisters in Christ - and even if they don't repent after they have been given the means to do so, God alone will be their Judge (see When Will You Be Judged? and They Shall Be Ashes).
"5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; 5:45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good [see also Why Does God Allow Evil?], and sendeth rain on the just and on the unjust. 5:46 For if ye love them which love you, what reward have ye? do not even the publicans the same? 5:47 And if ye salute [see Salutations] your brethren only, what do ye more than others? do not even the Publicans so? 5:48 Be ye therefore perfect [see Perfection], even as your Father which is in heaven is perfect." (Matthew 5:44-48 KJV)
The Ten Commandments (see the Fact Finder question below) are "briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself."
"13:8 Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law. 13:9 For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself. 13:10 Love worketh no ill to his neighbour: therefore love is the fulfilling of the law. 13:11 And that, knowing the time, that now it is high time to awake out of sleep [see Awaken To The Light]: for now is our salvation nearer than when we believed. 13:12 The night is far spent [see Could Christ Return Tonight?], the day is at hand: let us therefore cast off the works of darkness, and let us put on the armour of light." (Romans 13:8-12 KJV)
Loving one's neighbor (i.e. obeying the royal law of love for God and for fellow man) is vital to salvation. Those who refuse "shall not inherit the kingdom of God."
"5:14 For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself. 5:15 But if ye bite and devour one another [see Backbiting], take heed that ye be not consumed one of another. 5:16 This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
Fact Finder: (a) How can we truly love God? (b) How can we truly love our neighbors as ourselves?